During the past twenty years, a cheap and convenient method of producing paper printing plates has become popular. This has been developed as an off-shoot of the direct electrophotographic copying process (Electrofax) developed by RCA (see, for instance, U.S. Pat. No. 2,987,395), whereby paper, having a dye-sensitized zinc oxide resinous layer, is charged and light exposed to produce an electrostatic charge pattern. This pattern is then developed with a resinous carbon powder which may be, for instance, either liquid borne or carried triboelectrically in a magnetic brush.
In order to obtain customer acceptability, the copy produced by this process had to be white in appearance and of such a substance as to appear as similar to ordinary paper as possible. The white color was achieved by using a combination of differently colored sensitizing dyes.
In U.S. Pat. No. 2,735,784 there is described a process for producing a planographic printing plate in which a heavy duty paper is used, and there is no constraint on the color of the plate from the point of view of customer acceptability, making the choice of sensitizing dyes simpler. However, in order to use such a master for the purposes of printing, it is necessary that the print areas are ink receptive and the background areas water receptive. The former property is generally an outcome of the developer used in the copying process, but in order to achieve the water receptive/ink repellent properties of the background areas, it is necessary to treat the master after imaging with what has become known as a conversion etch. One such etch is for instance described in U.S. Pat. No. 3,714,891. Generally, use is made of a ferrocyanide solution to convert the zinc oxide on the surface of the master to the water-receptive zinc ferrocyanide.
Although it has been claimed that such plates may be made of plastic or even metal, this has not proven to be commercially successful. Plastic plates would be desirable over the paper plates because they would be more stable under printing conditions, as the plastic used could easily be water resistant. However, the necessity of having a conductive base material for the electrophotographic process to function has provided a considerable barrier to satisfactory development, as, in general, plastics do not have the low electrical resistance required. The disadvantage of the present paper plates is that they absorb water used in the printing process and this causes stretching and cockling and limits plate life.
When the process was first commercially exploited, it was possible to buy a machine which could be used as a copier and could also be used to make a printing master. However, parallel to the development of the direct electrophotographic process was the development of the indirect (Xerographic) process whereby the imaging is done onto a drum or continuous band of electrophotosensitive material such as amorphous silicon, and the image developed with a subsequent offset of the image onto plain paper. This process was preferred by customers because they wanted their copies on plain paper. The Electrofax process could only produce coated paper that did not feel like plain paper, was easily marked, for instance with a coin, and was at best off-white in appearance.
Consequently, with the growth in popularity of the plain paper copier, the Electrofax process declined to a more limited market of machines dedicated solely to reproducing paper offset masters. This is less desirable to the customer who often has to purchase and maintain both copying and plate-making machines, with the latter being more expensive because of their more limited market.
A further development in printing has occurred with the advent of the computer and especially the personal computer (PC) and the introduction of Desk Top Publishing. With appropriate software programs, it is possible to generate within the computer all kinds of graphical and typesetting designs for the purpose of subsequent printing, thus eliminating long hours of arduous preparation by hand. The digital information must be converted to hard copy which may subsequently be used to produce a printing plate. A suitable method that has been developed is to use the digital signal generated by the computer to modulate a laser beam as a means of imaging in the indirect or Xerographic process, and laser printers are now widely sold for this. The resulting hard copy is then used, either as an original on an electrostatic plate-maker, or to produce a transparent film that can be then used as a master for a metal offset plate.
While the advantages of generating an offset litho plate directly from digital information has been recognized in U.S. Pat. Nos. 4,149,798 and 4,774,532, these patents utilize the direct (Electrofax) electrophotographic plate-making method as described above. Therefore, the equipment needed must be dedicated to plate making, and the type of plates made is, in practice, restricted to paper plates. German Patent No. 2,726,263 describes an aluminum based printing plate electrophotographically worked using laser imaging, but the limitation of using dedicated equipment applies equally to this invention. Moreover, it involves a wash-off process to hydrophilize the background. German Patent No. 2,607,207 uses laser imaging to produce a printing plate, but this is a non-electrophotographic process.
In my previous patent application GB 2,110,161 A, there is described an offset plate that can be imaged directly on a plain paper copier. Such plates were designed to work primarily with a plain paper technology involving cold-pressure fusing of the image and development by cold pressure fusing. However, it has been found that the main problem in utilizing such plates is that, frequently during imaging, extraneous background dots appear on the plate.
While background dots are imperceptible to the human eye in the production of a final hard copy directly from the plain paper coper, if the copy is used as a printing plate, inevitable dot enlargement occurs during printing and such dots then appear as an undesirable, clearly visible background. It is this problem that would preclude the commercial exploitation of the idea described above, and even if the idea had occurred to anyone to apply such technology to laser printing, this problem would have precluded its consideration.
Therefore, it would be desirable to provide a method of economically producing printing masters, free of undesirable background, by use of commercially available plain paper reproduction equipment based on the electrophotographic process.